More about isotopes

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More about isotopes

• Atomic mass vs average atomic mass or atomic
  weight

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Atomic Mass

• The mass of a specific isotope of an element
  expressed in atomic mass units (amu)
• The atomic mass is the total mass of the protons,
  neutrons and electrons in the atom
• Atomic mass mass of an atom
• This is what we refer to in our isotope
  expression, eg.  

3
Average Atomic Mass (also called Atomic Weight)

• This is the number that you see in the periodic
  table.
• This is the weighted average of the all of the
  isotopes of Hydrogen

1 H 1.008
Atomic Number Symbol of element Atomic Weight
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What is a Weighted Average?

• The isotopes of an element do not occur with
  equal frequency or amounts
• For example, the relative abundances for the
  three carbon isotopes are
• carbon-12 98.9
• carbon-13 1.1,
• carbon-14 lt001.
• The "average" mass for the atoms of an element is
  dictated by the most abundant or common isotope

• The atomic weights in the periodic table
  weighted averages
• Tabulated atomic weight value doesn't match any
  actual atom, but is closer to the weight for the
  most common isotope
• Well follow the steps of how to calculate a
  weighted average next.

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Calculating Weighted Average

• weighted average ( decimal fraction A) mass A
  ( decimal fraction B) mass B

• Carbon-12 atomic mass 12 amu abundance 98.9
• Carbon-13 atomic mass 13 amu abundance 1.1
• Carbon-14 atomic mass 14 amu abundance
  0.001

Atomic weight (0.989 12) (0.011 13)
(0.00001 14) 12.011
6 C 12.011
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Average Atomic Mass Worksheet

• Homework - I will post answers on website
  Monday. You will have to show ALL of your work.